The subject disclosure relates to superconducting quantum circuits, and more specifically, to multi-mode qubit readout and qubit state assignment.
Quantum computing is generally the use of quantum-mechanical phenomena for the purpose of performing computing and information processing functions. Quantum computing can be viewed in contrast to classical computing, which generally operates on binary values with transistors. That is, while classical computers can operate on bit values that are either 0 or 1, quantum computers operate on quantum bits that comprise superpositions of both 0 and 1, can entangle multiple quantum bits (qubits), and use interference.
Quantum computing hardware is different from classical computing hardware. In particular, superconducting quantum circuits generally rely on Josephson junctions, which can be fabricated on a semiconductor substrate. A Josephson junction generally manifests the Josephson effect of a supercurrent, where current can flow indefinitely across a Josephson junction without an applied voltage. One or more Josephson junctions can be embedded in a superconducting circuit to form a quantum bit (qubit). A plurality of such qubits can be arranged in a superconducting quantum circuit fabricated on a semiconductor substrate, which can further comprise microwave readout resonators coupled to the respective qubits that facilitate reading quantum information of the qubits (also referred to as addressing or reading a quantum logic state of the qubit). Such a superconducting quantum circuit and microwave readout resonators can be integrated onto a semiconducting substrate to form an integrated quantum processor that can execute computations and information processing functions that are substantially more complex than can be executed by classical computing devices (e.g., general-purpose computers, special-purpose computers, etc.).
A problem associated with efficiently and rapidly reading out a superconducting qubit involves a number of design parameters often in conflict with each other. First, typically what is known as Quantum-non-demolition (QND) measurements are preferred so that the qubits can be used again in a particular computation or algorithm after the measurement. This restricts almost exclusively the readout technique to the different regimes present in the circuit Quantum Eletrodynamics (circuit-QED) architecture, in which a superconducting resonator is coupled, capacitively or inductively, to a superconducting qubit. The quantum state of the qubit affects the resonance frequency of the coupled resonator and a precise readout of the qubit can be attained this way.
Whereas fast readout is often preferred, a readout resonator strongly coupled to the environment results in lower qubit coherence via the Purcell effect, by which the qubit relaxes its energy via the resonator to the environment. To alleviate this mechanism, some existing superconducting quantum systems employ a variety of filters that allow photons at the resonator frequency to be transmitted through the readout lines while curbing the relaxation of the qubit. These filters can be bulky and are often accompanied by design fabrication constrains that can limit the control of the dynamics of each readout resonator.
In addition, the large coupling of the readout resonator to the environment that makes possible fast, high-fidelity qubit state assignment, makes the system extremely sensitive to noise at the resonator frequency, which dephases the qubit in detriment of its coherence time. Purcell filters only protect the qubit against decay but do not protect the resonator against dephasing noise.